Section 19. Clause fifth of section seven of chapter four shall not apply to this chapter. A special law relating to an individual city or town enacted subsequent to November eighth, nineteen hundred and sixty-six in compliance with clause (1) or clause (2) of section eight of Article LXXXIX of the Amendments to the Constitution shall be subject to amendment or repeal through the adoption, revision or amendment of a charter by such city or town in accordance with the provisions of sections three and four of said Article LXXXIX unless such special law specifically provides otherwise, and shall be subject to amendment or repeal by laws enacted by the general court in conformity with powers reserved for the general court by section eight of said Article LXXXIX of said Amendments.